It is known that the use of nonlinear optical media is effective in achieving wavelength conversion devices. In particular, in the case of using a noncentrosymmetric uniaxial crystal, it is essential, for the purpose of improving the nonlinear optical properties of the uniaxial crystal, to realize quasi-phase matching by achieving periodical inversion of crystallograhic orientation (polarity or polarization). Nitride semiconductors, such as gallium nitride (GaN), that have recently received more attention for semiconductor element application, etc. are also uniaxial crystals, and are expected to be utilized as nonlinear optical media. For this reason, the development of techniques for periodic inversion of polarity in nitride semiconductors is required.
So far there has been proposed a pattern formation process that controls polarity inversion and defines a region to be inverted by using molecular beam epitaxy (MBE) for GaN film growth.
Meanwhile, in the case of use of the metal organic chemical vapor deposition (MOCVD) method, which is more suitable for practical use, there has been a demand for a pattern formation process that can control polarity inversion and define a region to be inverted.
For example, Patent Literature 1 discloses a polarity inversion technique that form a semiconductor thin film with polarity inversion by growing a semiconductor thin film on a sapphire substrate treated with HNO3 using a growth mask.
Further, Patent Literature 2 discloses, as an example of a polarity inversion technique based on electron beam irradiation of a substrate, a polarity inversion technique in which a sapphire substrate is treated in hydrogen at high temperatures, irradiated with an electron beam, and then deposited with a semiconductor thin film using metal organic chemical vapor deposition. Furthermore, Non-patent Literature 1 discloses a polarity inversion technique developed by improving the electron beam irradiation method.